This invention is in the field of blends of copolymers of tetrafluoroethylene and perfluoro(alkyl vinyl ether) that are miscible in amorphous regions and can be miscible in crystalline regions as well.
The melting points and crystallization temperatures of tetrafluoroethylene-perfluoro(alkyl vinyl ether) copolymers (known as TFE/PAVE or PFA) and tetrafluoroethylene-hexafluoropropylene copolymers (FEP) are strongly influenced by the amount of PAVE or HFP comonomer in the copolymer. An increase in the amount of comonomer in the copolymer results in a decrease in the polymer melting point.
PAVE comonomers include perfluoro(propyl vinyl ether) (PPVE), and perfluoro(ethyl vinyl ether) (PEVE), and perfluoro(methyl vinyl ether) (PMVE). In comparison with TFE/PPVE, the copolymers TFE/PEVE and TFE/PMVE have certain advantages. For example, the PEVE and PMVE comonomers polymerize more rapidly than PPVE, and polymerization is easier to control. Furthermore, PEVE and PMVE are distributed more uniformly in the polymer molecules. These features make them preferable to PPVE when higher comonomer content is needed, as for example, when increased copolymer flex life is desired. However, with higher comonomer content than prior-art PPVE-containing PFAs, the TFE/PEVE and TFE/PMVE polymers have lower melting points and lower maximum service temperatures than TFE/PPVE PFAs. As a result, articles made from them have lower use temperatures.
Examples are known in which the physical properties of a fluoropolymer are optimized by admixture of other fluoropolymers. U.S. Pat. No. 5,041,500 discloses heterogeneous blends of FEP with TFE/PPVE. U.S. Pat. No. 5,179,167 discloses blending of low and high molecular weight FEP or TFE/PPVE. Attempts have been made to improve the blend properties by more uniform and intimate mixing of the components: Macromolecules vol. 28, p. 2781 (1995); Journal of Polymer Science: Polymer Physics vol. 37, p. 679 (1999)). Blends of polytetrafluoroethylene (PTFE) with TFE/PPVE and of FEP with TFE/PMVE were found to cocrystallize if the melt blends were rapidly cooled with liquid nitrogen. However, under cooling conditions characteristic of normal polymer melt processing, the polymers of the blends crystallized separately, as shown by their separate melting points in the blend.
There is a need for fluoropolymer blend compositions that remain intimately blended after cooling, as evidenced by single melting points and single xcex1-transition temperatures.
It has been discovered that different melt processible copolymers of tetrafluoroethylene and perfluoro(alkyl vinyl ether) can be mixed together to produce a fluoropolymer composition in which the components are miscible on a molecular level. In greater detail, the melt processible fluoropolymer composition of the present invention comprises: a) a first copolymer of tetrafluoroethylene and perfluoro(alkyl vinyl ether) wherein the alkyl group of the perfluoro(alkyl vinyl ether) has at least 3 carbons, and b) a second copolymer of tetrafluoroethylene and perfluoro(alkyl vinyl ether) wherein the alkyl group of the perfluoro(alkyl vinyl ether) has 1 and/or 2 carbons, wherein each said first copolymer and said second copolymer has crystalline and amorphous regions, at least the amorphous regions of said first copolymer and second copolymer being miscible with one another as revealed by said composition exhibiting a single xcex1-transition temperature.